You might need the mpeg-2 Playback Component. Here's info about that from Squated5
"The installer of the MPEG-2 Playback Component may refuse to install the component in Lion and Mountain Lion. Apple states the component is unnecessary in Lion and Mountain Lion, however MPEG Streamclip still needs it.
To install the component in Lion or Mountain Lion, please download MPEG Streamclip 1.9.3b8 beta above; inside the disk image you will find the Utility MPEG2 Component M. Lion: use it to install the MPEG-2 Playback Component in Lion or Mountain Lion. The original installer's disk image (QuickTimeMPEG2.dmg) is required.
The current versions of MPEG Streamclip cannot take advantage of the built-in MPEG-2 functionality of Lion and Mountain Lion. For MPEG-2 files you still need to install the QuickTime MPEG-2 Playback Component, which is not preinstalled in Lion and Mountain Lion. You don't have to install QuickTime 7.
Also read the information for Snow Leopard below."
MacPro 8-core 2.8GHz, 16 GB RAM, OS 10.8.4, QT10.1, Kona 3, Dual Cinema 23, ATI Radeon HD 5870, 24" TV-Logic Monitor, ATTO ExpressSAS R380 RAID Adapter, PDE enclosure with 8-drive 6TB RAID 5
FCS 3 (FCP 7.0.3, Motion 4.0.3, Comp 3.5.3, DVDSP 4.2.2, Color 1.5.3)
FCP-X 10.0.9, Motion 5.0.7, Compressor 4.0.7
Pro Tools HD 10 w SYNC IO & 192 Digital I/O, Yamaha DM1000, Millennia Media HV-3C, Neumann U87, Schoeps Mk41 mics, Genelec DSP Monitors, Prima CDQ120 ISDN